fix(flux): decouple modular flux from classic and qwenimage imports

[akshan-main]

What does this PR do?

Addresses Issue 6 of the flux review (#13584). The modular Flux blocks imported FluxPipeline._pack_latents and _prepare_latent_image_ids, the classic Kontext PREFERRED_KONTEXT_RESOLUTIONS constant, and two QwenImage modular helpers. This replaces those cross-imports with flux-modular-local copies, following decoders.py.

The coupling check from the issue returns nothing and the modular flux tests pass.

Fixes #13584

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? (Part of [meta issue] Systematic model/pipeline review findings / tracking #13656, finding from flux model/pipeline review #13584) Discussed on slack with Sayak
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? (refactor, covered by the existing modular flux tests)

Who can review?

@DN6 @sayakpaul @yiyixuxu

[github-actions]

Hi @akshan-main, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.

[sayakpaul]

Sorry I don't get it. What's the problem with the existing imports? These assumed that the modular pipelines were implemented w.r.t their standard counterparts. @yiyixuxu am I missing something?

[akshan-main]

Sorry I don't get it. What's the problem with the existing imports? These assumed that the modular pipelines were implemented w.r.t their standard counterparts. @yiyixuxu am I missing something?

Hey!
This came from issue 6 of the flux review (#13584), which explicitly flagged these and pointed to decoders.py keeping its helper local. #13708 independently did the same decoupling into a flux-local pipeline_helpers.py and even added a test asserting modular flux doesn't import from ...pipelines or ..qwenimage, so it wasn't just my reading.

You're right the standard-counterpart imports are the normal pattern (qwenimage, flux2, all do it), so the FluxPipeline and Kontext ones are fine. The one I'd still flag is flux/inputs.py importing from ..qwenimage, the only cross-family import in modular_pipelines. I can narrow this to just that, or close it if you and @yiyixuxu prefer it all as-is.

[yiyixuxu]

Sorry I don't get it. What's the problem with the existing imports? These assumed that the modular pipelines were implemented w.r.t their standard counterparts. @yiyixuxu am I missing something?

ohh, I agree with the agent review that it should not cross-import here because modular vs standard is a parallel relationship, #Copied from would be better if it works, but the PR fix seems ok too

[sayakpaul]

Let's try using "# Copied from ..."?

[sayakpaul]

Let's try using "# Copied from ..." where possible?

[akshan-main]

Let's try using "# Copied from ..." where possible?

the two qwenimage modular helpers now use # Copied from